Method of forming cadmium sulfide photoconductive cells



Sept. 7, 1,954 S v FORGUE 2,688,564

METHOD OF FRMING CADMIUM SULFIDE PHOTOONDUCTIVE CELLS Filed NOV. 22, 1950 Mmm.

ATTORNEY Patented Sept. 7, 1954 METHOD F FORMING CADIVIIUM SULFIDE PHUTOCONDUCTIVE CELLS Stanley V. Forgue, Granbury, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 22, 1950, Serial No. 197,019

7 Claims.

This invention is directed to photoconductive materials and specifically to the method of. making and the use of photoconductive cadmium sul de.

The photoconductive properties of cadmium sulfide have been well recognized for sometime. A photoconductive material is one which has the property of conducting an electric current when light falls on the material. Such a material is placed in contact with and between a pair of electrodes connected in series with a source of potential. When no light falls upon the photoconductive material or in the dark, the conductivity of the material is of a certain value. When light is caused to strike the photoconductive material, it is observed that the conductivity of the material changes and results in an increased current ow between the two electrodes.

The photoconductive property of cadmium suliide material has been well known for sometime. The electric conductivity of the material in its pure state and in a crystalline form has been observed to be extremely good. However, the use of cadmium sulde as a photoconductor has been limited due to diii'iculties in producing steady photocurrents, as well as, the fact that cadmium sulfide possesses a relatively high conductivity in the dark. This last property of the material precludes its use in devices or systems where the dark current flow of the cell must be kept low. It is desirable that photoconductive cells exhibit a high resistivity to an electric current in the dark to prevent current losses during periods of time When the device is not being operated.

Also, to be effectively used for example in a television pick-up tube target, a photoconductive material must exhibit suicient dark resistivity to provide storage of a charge in the dark. The usual low dark resistivity of cadmium sulfide material severely limits it being used in pick-up tubes. A photoconductive pickup tube comprises an electron gun providing an electron beam for scanning the surface of a photoconductive target. The photoconductive target of the tube includes a transparent support member having a metallic or conductive film on the side of the support member facing the electron gun. The cadmium sullide is put down as a thin coating over the conductive film. The scanning electron beam establishes a charge potential on the surface of the photoconductive film by knocking out secondary electrons which are captured by a collecting electrode. The conductive film may have a voltage applied to it and thus maintain a difference of potential between the opposite surfaces of the photoconductive lm. Light striking the photoconductive film renders the film conductive and causes a now of current through the photoconductive film to bring the potential of the scanned surface towards the potential of the conductive film. The electron beam on scanning over the target surface will discharge the target in those areas illuminated by light. In pick-up tube applications, it is necessary that the charge on the scanned surface of the photoconductive film be maintained unchanged in the non-illuminated target areas. That is, it is important that there be substantially no flow of current inthe dark areas of the target between the surfaces of the photoconductive film. Such a condition provides a charge differential between the dark areas and the light areas of the target which can be detected by the beam scansion over the target surface.

It is therefore, an object of my invention to provide an improved cadmium sulde photoconductive material.

It is another object of my invention to provide a cadmium sulfide photoconductive material having optimum resistivity in the dark.

It is another object of my invention to provide an improved cadmium sulfide photoconductive material having improved sensitivity.

It is an additional `object of my invention to provide a photoconductive device using a cadmium sulfide photoconductive material having a high resistivity in the dark.

The invention specifically is directed to a method of forming a cadmium sulfide photoconductive material and to a light sensitive device utilizing such material. The specific method of making the cadmium sulfide photoconductive material is that of forming a nlm of cadmium sulde on a supporting member from the evaporation or sublimation of cadmium sulde. The cadmium sulfide film is then baked in an atmosphere containing oxygen for more than ten minutes and between a temperature of y250" centigrade to 650i' centigrade. The baking of the photoconductive material increases the dark current resistivity by substantially five orders of magnitude.

The novel features which I believe to be characteristic of my invention are set forth with par ticularity in the appended claims, but the invention it is best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

Figs. 1 and 2 disclose light sensitive devices using the cadmium sulfide material of my invention;

Figs. 3 and 4 schematically show apparatus for forming cadmium sulfide films on a supporting structure;

Fig. is a graphical showing of typical curves of cadmium sulfide material processed in accordance with my invention.

Fig. 1 discloses a photoconductive cell of somewhat conventional design. There is shown a supporting member I0 preferably of insulating material, such as glass for example. On one surface of supporting member I0, there is disposed a photoconductive film I2 such as cadmium sulfide. A pair of spaced electrodes I4 and IB are formed in contact with the photoconductive film I 2. The electrodes I4 and I6 either may be metal strips pressed against the photoconductive film I2 to make contact therewith or may be formed by spraying, sputtering, evaporating, or painting metal on the surface of photoconductive film I2. One successful method is to paint silver electrode contacts I4 and I6 on the surface of the cadmium sulfide lm I2. Electrodes I4 and I6 are connected in a circuit with a source of potential by leads I8 and in a manner such that a potential difference is established between electrodes I4 and I6. The nature of the cadmium sulfide film I2 is such that when light falls on film I2, the potential difference between electrodes I4 and I6 will cause a flow of current to take place.

Fig. 2 shows another form of a photoconductive cell known as a body type cell in which the current flow through the photoconductive material is through a thickness of the material rather than across the surface. In Fig. 2 a transparent supporting member 20 such as glass is used. On one surface of the glass support 20 is formed a conductive film 22, such as a transparent metal film formed by evaporation, for example. The conductive film 22 may also be of a transparent non-metallic material such as a coating of tin oxide, for example. On the exposed surface of conductive film 22, there is put down, or formed, a cadmium sulfide photoconductive nlm 24. And in contact with the exposed surface of iilm 24 is mounted a conductive electrode plate or film 26. Conductive films 22 and 2S are connected into a' circuit with a source of potential to provide a potential diierence across the photoconductive film 24. In a manner similar to the operation to the cell of Fig. 1, light passing through transparent layers 20 and 22 of the cell will strike the photoconductive nlm 24 and cause a now of current therebetween.

The resistivity of the cadmium suliide films I2 and 22 is less in the light than in the dark. However, a property of cadmium suliide which makes it poor photoconductive material is its relatively low dark resistivity. It is obvious that if a current continues to ow between electrodes I4 and I6 at all times and even in the dark that the uses of such a cell shown in Fig. 1 is limited. It is desirable that a photocell be operative or conductive only when exposed to light. In photoconductive pick-up tubes a dark resistivity of 101o or 1011 ohm-centimeters is desirable. Pure cadmium sulfide films, cadmium sulfide crystals, as well as sublimed or evaporated films of cadmium sulfide have consistently fallen far short of this resistivity although the photoconductivity of cadmium sulfide has always been high. Because of the relatively low dark resistivity of cadmium sulfidel photoconductive material, the material has never been fully utilized, especially in television pick-up tubes.

In accordance with my invention, I have found that the photoconductive properties of cadmium sulfide material can be greatly improved if an evaporated or sublimed film of cadmium sulfide is baked in an atmosphere containing oxygen. A cadmium sulde material processed according to my invention has an increased dark resistivity by 4 to 5 orders of magnitude.

Improved photocells of the type shown in Figs. 1 and 2 may be made in the following manner. The cadmium sulde film is put down on the supporting member I0 (Figure 1) directly, or upon the supporting member 20 (Figure 2) which has been previously coated with the conductive film 22. For coating with cadmium sulfide, the support member is placed in a tubing or an appropriate chamber 30 as shown in Fig. 3. A ceramic boat 32 with pure cadmium sulfide material is also placed within container 30. As indicated in Fig. 3, hydrogen gas is passed through the tube, first to drive out all the air and then to form a hydrogen atmosphere for promoting the coating of glass target support plate IU with a cadmium suliide film. The container 30 is surrounded with heating coils 33 and 34. The coils are closer together in the region surrounding boat 32 to provide a high temperature of substantially 1000 C. At this temperature, the cadmium sulfide tends to sublime into the hydrogen atmosphere. The heater coils are spaced farther apart in the region of target plate I0 so that the temperature in this region is substantially lower and between 'ZOO-800 C. The sublimed sulphide will tend to collect or condense within this cooler region and upon the target plate I0. It is not entirely clear what the function of the hydrogen atmosphere is. It is believed however, that to some extent the flow of hydrogen gas physically conveys the sublimed cadmium sulfide into the cooler region of the target plate I0. However, also a chemical reaction takes place in which the hydrogen combines with the sulfur of the cadmium sulde to form hydrogen sulfide gas and sublimed cadmium metal. The sublimed metal condenses or deposits on the target plate IIJ and then recombines chemically with hydrogen sulfide gas present to form a cadmium sulfide layer on the surface of the coated target sheet I0. These chemical reactions, which are believed to take place in container 3D, permit the sublimation of the cadmium sulfide and formation of the target film at a lower temperature.

A sublimed cadmium sulfide target film has been formed by the use of nitrogen gas but only by heating the cadmium sulfide boat to some 200 C. higher than that needed with the hydrogen atmosphere. The cadmium sulfide film may also be formed on the supporting member I0 (or 20) by the evaporation of cadmium sulnde material. The support member I0 is placed within a bell jar 38 (Figure 4), which is then evacuated in any well known manner such as by a diffusion pump as shown. Also within the bell jar and spaced from the support member I0 is placed a small crucible 40 comprising a filament wire wound in a manner to form a cup for holding a quantity of cadmium sulfide material. The iilament wire 42 can be tungsten coated with aluminum oxide to separate the cadmium sulfide material from the wire. Bell jar 38 is evacuted to a pressure in the order of 10-5 mm. to 10-8 mm. of mercury. Current is passed through the filament wire 42 to heat the cadmium sulfide within the crucible 40 to a temperature of about 830 C. At

this temperature, it can be observed that cad.- mium sulde is being evaporated from the crucible and being deposited upon the cooler support member I0. Evaporation of the cadmium sulfide is continued until the thickness of the deposit on support member I0 is usually some- Where between 0.01 mil and 1 mil depending upon the particular application in which the surfaces are to be used. For example, a typical thickness for a television pick-up tube, when capacity time lag is to be avoided is about 0.1 mil, while for a storage tube, 0.01 mil or thinner would be desirable. The thickness of the cadmium sulfide film can be determined by well known opf tical interference methods, when the films are very thin, or by a depth microscope when the films are greater than about 0.1 mil thick. .When the cadmium sulfide film has reached an optimum thickness on the support member I0, the evaporation of the material is stopped and the coated support member removed from the -bell jar 38.

To provide the required higher dark resistivity for the cadmium sulfide material, I have found it is necessary to bake the cadmium sulfide target in an atmosphere containing oxygen for more than about 10 minutes and at a temperature between 250 C. and 650 C. This oxygen baking of a cadmium sulfide target increases the resistivity of the cadmium sulfide in the dark from a value somewhat less than 106 ohm-centimeters to above the minimum required value of 1010 or 1011 ohm-centimeters for a pick-up tube of the type described above.

Figure 5 shows the typical results obtained by such oxygen baking of many samples of cadmium sulfide target material. Curve 46 of Figure 5 shows the photoconductive current of cadmium sulfide material under illumination, while curve 48 represents the dark current of the photoconductive material. The current values given along the vertical axis of the graph are arbitrary units. The temperatures indicated along the horizontal axis of the graph are those at which the material was subjected during a series of oxygen or air bakes, after which the material was cooled to room temperature and the light and dark current values of the curves obtained. It is noted that at first as the temperature of the air bake increases, there is very little change between the dark current and light current values, until the material is baked at a temperature between 400 C. and 500 C. Within this temperature range and as indicated by the curves, the light current value of the cadmium sulde material typically decreases slightly while the value of the dark current of the same material takes a decided drop, of some 5 or more orders of magnitude from the original value of the dark current.

The current flow indicated in Figure 5 is also an indication of the resistivity of the material, but in an inverse sense. Figure 5 indicates that baking a cadmium sulfide material in oxygen and between 400 C. and 600 C. will increase the resistivity of the material by substantially 5 orders of magnitude or more. Although it is not indicated in Figure 5, some samples have shown a decided improvement in dark current resistivity after being baked at 250 C., while other samples have improved dark current resistivity after baking at substantially 650 C. From baking many samples, it can be said that the dark resistivity of cadmium material starts to increase after 250 C. and is also improved by baking at 6 650 C. The optimum baking temperature range is however between 40.0 C. and 500 C.

The actual process of baking the target electrode is relatively simple. The photocathode surface formed as described above is placed in an oven and baked at around 450 C. for a period of time between 5 minutes and 1/2 hour. The atmosphere of the oven may be air or oxygen. The target is then mounted in the tube structure and the tube processed in the conventional manner.

The oxygen content of the normal atmosphere is sufficient to activiate the cadmium sulfide material in the time specified. However, activation may be carried out with a greater or less percentage of oxygen. For example, successful activation has taken place with a pure oxygen atmosphere. It is only necessary that the baking take place sufficiently long and with sufficient oxygen in the atmosphere to complete activa.- tion. A longer baking time than necessary will not produce any ill effects.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. The method of forming a photoconductive film on a supporting member, said method comprising the steps of, depositing a thin film of cadmium sulfide on said supporting member by sublimation, baking said cadmium sulfide film for more than five minutes in an atmosphere containing oxygen to increase the resistivity of the film in the dark.

2. The method of forming a photoconductive iilm on a supporting member, said method comprising the steps of, evaporating cadmium sulfide material in a vacuum, condensing the evaporated cadmium sulfide material as a thin film on a surface of said supporting member, oxidizing said film by baking said cadmium sulfide film for more than five minutes in an atmosphere containing oxygen and at a temperature between 250 centigrade and 650 centigrade until the resistivity of the film in the dark has been increased.

3. The method of forming a photoconductive film on a supporting member, said method comprising the steps of, evaporating cadmium sulfide material in a vacuum, condensing the evaporated cadmium sulfide material on a surface of said supporting member as a thin film, baking said cadmium sulfide film for more than five minutes in an atmosphere containing oxygen and at a temperature between 400 centigrade and 500 centigrade to activate said film and increase the resistivity of the film in the dark.

4. The method of forming a photoconductive film on a supporting member, said method comprising the steps of, depositing a thin film of cadmium sulfide on said supporting member by sublimation in a hydrogen atmosphere, oxidizmember by .sublimation a film of cadmium sulfide having a thickness between 0.01 mil and 1.0 mil, activating said cadmium sulfide film by baking 7, said cadmium sulde lm for more than ten minues in an atmosphere containing oxygen.

6. The method of making a. photoconductive cell on the surface of a support member, said method comprising the steps of, arranging said support member surface and an amount of cadmium sulfide material in a hydrogen atmosphere, heating said cadmium sulde material to sublimation temperature, controlling the temperature of said support surface to deposit a thin lm of said sublimed cadmium sulfide on said support member surface by condensation, oxidizing said cadmium sulde film by baking said photoconductive lm for about ve minutes in an oxygen atmosphere until the resistivity of the lm in the dark is increased.

'7. The method of making a photo-conductive electrode for a pickup tube, said method comprising the steps of, providing a conductive iilm on one surface of a support member, arranging said support member surface and an amount of cadmium sulfide material in a hydrogen atmosphere, heating said cadmium sulfide material to sublimation temperature, controlling the temperature of said support surface to deposit a thin film of said sublimed cadmium sulde on said support member surface by condensation, activating said cadmium sulde film by baking said photoconductive film for about ve minutes in an oxygen atmosphere to increase the resistivity of the lm in the dark.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,711,974 Snelling May 7, 1929 2,281,474 `Cartwright et al. Apr. 28, 1942 2,398,382 Lyon Apr. 16, 1946 2,448,516 Cashman Sept. 7, 1948 FOREIGN PATENTS Number Country Date 521,099 Great Britain May 13, 1940 

1. THE METHOD OF FORMING A PHOTOCONDUCTIVE FILM ON A SUPPORTING MEMBER, SAID METHOD COMPRISING THE STEPS OF, DEPOSITING A THIN FILM OF CADMIUM SULFIDE ON SAID SUPPORTING MEMBER BY SUBLIMATION, BAKING SAID CADMIUM SULFIDE FILM FOR MORE THAN FIVE MINUTES IN AN ATMOSPHERE CONTAINING OXYGEN TO INCREASE THE RESISTIVITY OF THE FILM IN THE DARK. 